This invention relates generally to ceramic crucibles and more particularly to zirconia crucibles used in vacuum induction furnaces for melting refractory metals.
Vacuum electric induction furnaces are usually used for the melting and casting of special metals and alloys which require a high temperature and inert conditions to preserve the purity of the metal. Such metals are refractory super alloys which are nickel or cobalt based or precious metals such as platinum. These metals are generally melted in small quantities using electric induction coils as the heat source, since the induction process heats only the metal itself and any heating of the containing crucible is by conduction from the molten metal.
Furnaces of this type usually use a dual vacuum chamber construction with an upper melting chamber and a lower mold chamber which are separated by an interlock door. This door is closed while the charge in the crucible is being heated to allow mold assemblies to be replaced. When the melting is complete, the interlock door is opened, the mold assembly raised to a charging position, and the crucible assembly tilted toward a horizontal position to pour the contents into the mold assembly. After the pour is completed, the mold assembly is lowered and the interlock door closed. The crucible is then refilled with a billet through a billet charging door from a billet chamber, which is also under vacuum conditions, after which the crucible assembly is returned to a vertical position and the new billet melted.
During the cycle only the mold chamber and the billet chamber require cycling of the atmosphere to draw a new vacuum each time. The melt chamber remains essentially at vacuum during the whole cycle except for any leakage. However, periodically it is necessary to replace the crucible, and this has heretofore required the shutting down of the operation, and the introduction of the atmosphere into the melting chamber.
These furnaces have usually used crucibles having at least a liner made of zirconium oxide stabilized by a small addition of magnesium oxide or calcium oxide because of its highly desirable properties including its resistance to erosion, non-wetting by the molten metal, resistance to thermal shock, and low thermal conductivity. However, such crucibles are of relatively thin wall thickness because increasing the wall thickness tends to shorten the life of the crucible since thick walls tend to crack under the thermal cycling inherent in the melting process as the relatively cold billet is heated to the melting temperature.
Because of this limitation on the wall thickness, it has been necessary to provide a physical support of the crucible which is employed only as a relatively thin liner supported by a packing of crushed ceramic material such as granular aluminum oxide or granular zirconium oxide around the zirconia crucible and contained within an outer support made of a less refractory material such as a cement or insulating material. The replacement of a crucible liner with this construction has required the shutting down of the vacuum furnace and the removal of the crucible assembly. When this is done, the old crucible is removed together with the packing material which being loose granular material results is a difficult operation which is complicated by the replacement of the same construction. Also, the granular material is undesirable due to dusting problems, which can be a health hazard. This has meant that whenever it has been necessary to replace the crucible liner during operation of the furnace, the entire furnace operation must be shut down and placed out of production for an undesirable period of time. As an example, during production runs a melting cycle from pour to pour may take only about six to eight minutes, and assuming that a crucible liner has a life of 60 to 90 cycles, the crucible liner must be replaced every six to twelve hours. It can therefore be seen that an extended down time for the replacement of the crucible liner can cause a significant reduction in productivity for the furnace.
It has been recognized that the down time for liner replacement is caused primarily by the need for the use of the granular material for the crucible support. If the packing material could be combined with the liner as a package, the time for liner replacement could be greatly reduced. It has been proposed that instead of using loose granular material, a supporting capsule of castable ceramic such as aluminum silicate or oxide could be cast around the liner as a wet slurry, then dried and fired at a high temperature to form a unitary structure which could then be easily used to replace the existing structure within the induction coils. Such an arrangement has been disclosed in U.S. Pat. No. 4,160,796 granted Jul. 10, 1979. However, such an arrangement has not met with commercial success, apparently because the resultant structure did not have the required resistance to thermal shock under actual operating conditions to provide a significant improvement in the overall operation of the furnace.